Field of the Invention
This invention relates generally to methods for correcting dead-reckoning navigation errors in vehicles which can move past an area having a patterned position marker. In particular, the area can be on a horizontal floor, such as a warehouse floor or a table top; or it can be on a wall, a ceiling, or some other surface. The present invention describes a class of patterns to be used for markers, and methods that an autonomous robot or other vehicle can use to analyze the patterns for determining coordinates to be used for navigational corrections.
The purpose of this invention is providing a simple, single-sensor, low-cost, highly-accurate system for correcting navigation errors as a vehicle moves without changing direction along a dynamically variable path which crosses one or more two-dimensional patterned markers.
The Prior Art
Mobile robots commonly use odometry for position estimation. From wheel rotation measurements a robot can estimate its direction and travel distance along its path, and use those estimates to approximate its position. It is well known that this dead-reckoning navigation suffers from accumulation of errors, so the use of dead-reckoning for extended travel needs a method for correcting the estimated position. Many methods are known.
For example, a U.S. Pat. No. 4,541,049 by S. H. N. Ahlbom, et al. derives corrections using signals from a vehicle-mounted set of optical sensors arranged in a line as the vehicle crosses known-position lines on the floor. The present invention is simpler in that it uses a single optical sensor.
The robot known as “Odin” (reference: “Odin, a robot for odometry”) uses a related method: the robot optically senses one leg of a known-position L-shaped pattern as it moves into the angle between the two legs, then it turns to cross and sense the other leg; it uses the positions sensed to determine its location. The present invention does not need to make large path direction changes to get the data necessary for navigation corrections.
U.S. Pat. No. 4,847,773 by C. C. van Helsdingen, et al. describes a grid of “passive” markers over which a sensor-equipped vehicle moves, but it does not describe markers with patterns similar to those of the present invention.
U.S. Pat. No. 5,111,401 by H. R. Everett, et al teaches the use of a stripe with position markers at various points; the robot moves until it encounters the stripe, then follows the stripe until it encounters a position marker, which it uses to establish its location by referring to predetermined coordinates of the position markers. The position markers are wider places on the stripe. The patent mentions that the “simplistic marker pattern” can be replaced with a more complex pattern providing unique identification of the marker, but it does not provide a detailed description of any such complex pattern. The present invention does not need stripe following, and it has complete descriptions of position markers.